Vehicle speed measurement and enforcement systems can be employed to monitor traffic and identify vehicles moving faster than the posted speed limit. Such vehicle speed measurement systems can employ ALPR (Automatic License Plate Recognition) systems to determine an accurate speed of a vehicle. ALPR (Automatic License Plate Recognition) system often functions as the core module of “intelligent” transportation infrastructure applications. License plate recognition can be employed to identify the vehicle by automatically reading a license plate utilizing an image processing and character recognition technology. A license plate recognition operation can be performed by locating the license plate in an image, segmenting the characters in the plate, and performing an OCR (Optical Character Recognition) operation with respect to the characters identified.
The majority of prior art vehicle speed measurement systems utilize one or more single image capturing units (e.g., camera) to measure the speed by calculating a distance traveled by the vehicle over multiple frames. The time between frames is known and often fixed so that the speed can be calculated by dividing the distance traveled over time. The vehicle can be tracked utilizing features of the vehicle and the movement of the features can be employed to estimate the speed. Consequently, several problems arise when multiple vehicles enter the field of view of the image capturing unit at similar time traveling at similar speed.
One such problem is that two vehicles and their associated features appear as a single vehicle thus leading to obvious errors. A solution to this problem involves identifying and tracking the vehicle license plate as a relevant feature. Furthermore, if a speed violation is detected a plate code is required to issue the citation. Another problem is that the height of the features changes from vehicle to vehicle due to variety of the vehicle height, shape, as well as location of the license plate, if the license plate is utilized for tracking. Such varying feature heights with respect to the road surface for fixed camera installation change the effective distance that the feature travels for a fixed speed from a reference point of the pole mounted camera thus adding error to the speed measurement. An extra speed sensor such as, for example, radar, laser, multiple cameras (stereoscopic), etc., can be employed to solve the problem, however, such extra sensors add unnecessary cost and complexity.
Based on the foregoing, it is believed that a need exists for an improved vehicle speed measurement method and system. A need also exists for an improved method for measuring height of a license plate from a road surface, as will be described in greater detail herein.